Heat Tolerance of Some Animals Is Already Exceeded

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Tiistai 19.4.2022 klo 20:22 - Mikko Nikinmaa

Among animals, humans are probably among the best in adapting to hot and arid environments. There are several reasons for this. First, we have lost the fur coat almost completely – thus the name “naked ape”. This maximizes the heat loss. Second, humans are able to use evaporative cooling better than virtually any animal. We have more sweat glands per unit area than other mammals, and the glands are of the eccrine type, whereas the glands of most other mammals are of apocrine type. Eccrine glands are capable of secreting sweat of low salinity for a long period of time. In contrast, the sweat form apocrine glands is of higher salinity, and the secretion capacity is smaller. Many mammals such as cats and dogs have limited number of sweat glands: they are completely absent in bats. Also in birds evaporative water loss is a major mechanism of cooling. This is often done by panting and in some species by gular vibration. The physiology of heat tolerance in small endotherms has been reviewed by McKechnie and Wolf in Physiology 34:302-313 (2019).

The problem with evaporative cooling is that it requires water, which is very scarce in arid environments. Thus, even in species such as humans, which have efficient evaporative heat loss, the lack of water may cause mortalities. As most hot areas also become drier, this is a major problem with climate change. That the heat-related problems have already become serious ones even to humans is given by the estimated increase in heat-related deaths, which has increased 20-fold in 20 years after 1990’s. For humans the mortality is small as compared to bats in Australia and in South Africa. As bats don’t have sweat glands, their tolerance of increased temperature is very weak. So heat waves have caused many bats to literally drop dead from the trees they have lived in. It is estimated that for one particular species 1/3 of the population has died because of the heat. In Australia and South Africa also mass mortalities of birds have occurred because of heat waves. I bet that similar heat-induced mass mortalities have taken place also in other hot, arid areas, but there have not been scientific reports about them. Also birds appear to have weaker tolerance of increased temperatures than mammals.

Any temperature increase now will put an increasing number of species to knife’s edge regarding their survival. Bats and birds serve as first indicators of intolerable heat. So, canary birds were used in coal mines as warning signs. Now birds indicate that coal use must be stopped. It is appropriate that the first mass mortalities of animals occurred in Australia as the country is the biggest coal exporter in the world.

Avainsanat: climate change, temperature regulation, mass mortalities, evaporative cooling

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